Topic
Engine and emissions codes (P0)
P codes come from the engine and transmission computers. Read the subsystem digit first: it tells you whether you are looking at fuel and air, ignition, emissions, idle, the computer itself or the transmission.
- Lamp
- check engine (malfunction indicator lamp)
- Code format
- P + 4 characters, e.g. P0420
Powertrain codes start with P. A second character of 0 means a generic code with the same meaning on every vehicle; 1 means the maker defines it. The third character names the subsystem: 1 and 2 are fuel and air metering, 3 is ignition and misfire, 4 is auxiliary emissions such as EGR, EVAP and the catalyst, 5 is speed and idle control, 6 is the computer and its outputs, and 7 to 9 are the transmission.
A code says which test failed, not which part to replace. P0171 (system too lean, bank 1) can come from a vacuum leak, a dirty mass airflow sensor or weak fuel delivery. Use the freeze frame and live data, especially fuel trims and oxygen sensors, to narrow it down before buying parts. The fuel trim reader helps with lean and rich codes.
A steady check engine light means a fault that should be checked soon. A flashing light means a misfire bad enough to damage the catalytic converter: reduce load and get it fixed before driving further.
Common mistakes
- Replacing the part named in the code description without testing it.
- Fixing a downstream code such as P0420 before the misfire or fuel trim code that caused it.
- Clearing the code before saving the freeze frame data.
Pages on this topic
- Why is my check engine light on? The common causes in orderWhy is your check engine light on? The most common causes from a loose gas cap to misfires, what the light means, and how to find the real reason with a scanner.Explainer
- P2263 code: Turbo/supercharger boost system performanceP2263 means actual boost does not match what the engine computer expects. Learn the common causes, how to test for boost leaks, and how to fix it.Fix
- P0705 code: Transmission range sensor circuitP0705 means the computer got an invalid signal from the transmission range sensor. See the symptoms, causes, how to test it and the safety risks of driving.Fix
- P0606 code: Control module processor faultP0606 means the engine computer found an internal processor fault. Learn the common causes, from low voltage to software, how to diagnose it, and next steps.Fix
- P0496 code: EVAP flow during non-purge conditionP0496 means fuel vapor is flowing to the engine when the purge valve should be closed. Learn the causes, how to test the purge valve, and how to fix it.Fix
- P0456 code: Evaporative emission system small leak detectedP0456 means the EVAP system found a very small vapor leak. Learn the common causes, how to find the leak, and whether it is safe to keep driving.Fix
- P0455 code: Evaporative emission system large leak detectedP0455 means the EVAP system found a large leak. Learn the common causes, starting with the gas cap, how to diagnose it, and how urgent it is.Fix
- P0449 code: EVAP vent valve/solenoid circuitP0449 means the computer sees an electrical fault in the EVAP vent valve circuit. See the symptoms, common causes, how to test it, and how to fix it.Fix
- P0446 code: EVAP vent control circuitP0446 points to the EVAP canister vent valve or its circuit. Learn what it means, the common causes, how to diagnose it with a scan tool, and how to fix it.Fix
- P0442 code: Evaporative emission system small leak detected (0.040 in)P0442 means the EVAP system found a small leak. Learn the common causes, how to find the leak, what a scan tool shows, and whether it is safe to drive.Fix
- P0420 vs P0430: bank 1 and bank 2 catalyst codesP0420 and P0430 mean the same thing on different sides of the engine. How to find bank 1 and bank 2, what causes each code, and what to check first.Comparison
- P0420 code: Catalyst system efficiency below threshold (bank 1)P0420 means the bank 1 catalytic converter is not cleaning exhaust as well as it should. Causes, how to diagnose it, how to fix it, and whether you can drive.Fix
- P0401 code: EGR flow insufficientP0401 means the engine computer saw too little exhaust gas recirculation flow. Common causes like carbon-clogged passages, how to diagnose it, and how to fix it.Fix
- P0340 code: Camshaft position sensor A circuit (bank 1)P0340 means the computer is not getting a valid signal from camshaft position sensor A on bank 1. See the symptoms, causes, tests and whether you can drive.Fix
- P0335 code: Crankshaft position sensor A circuitP0335 means the engine computer has a problem with the crankshaft position sensor signal. Learn the symptoms, causes, how to test it, and if you can drive.Fix
- P0325 code: Knock sensor 1 circuit (bank 1)P0325 means a problem in the knock sensor 1 circuit. Learn the symptoms, common causes, how to test the sensor and wiring, and whether you can keep driving.Fix
- P0301 code: Cylinder 1 misfire detected (also P0302-P0306)P0301 to P0306 mean a misfire in one specific cylinder. Learn how to read the cylinder number, run a swap test, find the cause and fix it safely.Fix
- P0300 code: Random/multiple cylinder misfire detectedP0300 means the engine is misfiring in more than one cylinder or at random. See the causes, how to diagnose it, and why a flashing light means stop driving.Fix
- P0299 code: Turbocharger/supercharger underboostP0299 means the turbo or supercharger is making less boost than the engine computer expects. Causes, how to diagnose boost leaks, fixes, and driving advice.Fix
- P0174 code: System too lean (bank 2)P0174 means bank 2 is running lean and fuel trims have maxed out. Common causes, how to read fuel trims to find the problem, and how to fix it.Fix
- P0172 code: System too rich (bank 1)P0172 means bank 1 is running rich and the computer cannot pull enough fuel. Learn the symptoms, common causes, how to read fuel trims, and how to fix it.Fix
- P0171 code: System too lean (bank 1)P0171 means bank 1 is running lean. Learn what causes it, how to read fuel trims to find the problem, how to fix it, and whether it is safe to drive.Fix
- P0141 code: O2 sensor heater circuit (bank 1 sensor 2)P0141 means the heater in the downstream O2 sensor on bank 1 has a circuit problem. What it means, common causes, how to test it, and how to fix it.Fix
- P0135 code: O2 sensor heater circuit (bank 1 sensor 1)P0135 means a fault in the upstream O2 sensor heater circuit on bank 1. Learn the causes, how to test the fuse, heater and wiring, and how to fix it.Fix
- P0128 code: Coolant thermostat below regulating temperatureP0128 means the engine takes too long to warm up, usually from a thermostat stuck open. Learn the symptoms, causes, how to test it, and how urgent it is.Fix
- P0118 code: Engine coolant temperature sensor circuit highP0118 means the coolant temperature sensor signal is too high, so the PCM sees a very cold engine. Learn the causes, how to test it, and how to fix it.Fix
- P0101 code: Mass air flow sensor circuit range/performanceP0101 means the MAF sensor reading does not match what the computer expects. See the symptoms, common causes like a dirty MAF or air leak, tests and fixes.Fix
- P0016 code: Crankshaft/camshaft position correlation (bank 1 sensor A)P0016 means the crank and cam position signals do not line up on bank 1. Learn the causes, from low oil to timing chain wear, and how to diagnose it.Fix
- P0014 code: Camshaft position B timing over-advanced (bank 1)P0014 means the bank 1 exhaust camshaft is more advanced than the computer commanded. Learn the causes, from low oil to a stuck VVT solenoid, and how to fix it.Fix
- P0011 code: Camshaft position A timing over-advanced (bank 1)P0011 means bank 1 intake cam timing is more advanced than commanded. Learn the causes, from low oil to a sticking oil control valve, and how to fix it.Fix
- How to read OBD2 codes and what each character meansLearn how to read OBD2 codes: what the letter and four digits mean, generic vs manufacturer codes, and how to look up any code before you buy parts.How-to
- Fuel trims explained: short term, long term and what's normalFuel trims explained: what short term and long term fuel trim mean, what numbers are normal, and how to use them to find vacuum leaks and fuel problems.Explainer
- Freeze frame data explained: what the snapshot tells youFreeze frame data is the snapshot your car saves when a code sets. Learn what each value means and how to use RPM, load, temperature, and fuel trims to diagnose.Explainer
- Check engine light solid vs flashing: what each meansA solid check engine light means a fault was stored. A flashing light means an active misfire that can damage the catalytic converter. What to do for each.Comparison
- Check engine light on but the car runs fineCheck engine light on but the car runs fine? Here are the usual causes, how to read the code, and when it is still worth fixing soon even with no symptoms.Fix
- Check engine light after getting gas: the gas cap and EVAP codesCheck engine light after getting gas? A loose or worn gas cap often sets an EVAP leak code. How to confirm it, what codes to expect, and when it is something else.Fix
- Can you drive with the check engine light on?Can you drive with the check engine light on? Solid usually means drive carefully and scan soon; flashing means pull over. What to check and what to do next.Explainer